Ferritic stainless steel sheet for urea scr casing (as amended)

ABSTRACT

Provided is a ferritic stainless steel sheet for a urea SCR casing which is excellent in terms of corrosion resistance in a urea SCR environment and which increases the durability of a urea SCR casing. The steel sheet has a chemical composition containing, by mass %, C: 0.020% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.01% or more and 0.50% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.01% or more and 0.20% or less, Cr: 20.5% or more and 24.0% or less, Cu: 0.40% or more and 0.80% or less, Ni: 0.05% or more and 0.6% or less, N: 0.020% or less, one or both selected from among Ti: 0.01% or more and 0.40% or less and Nb: 0.01% or more and 0.55% or less, and the balance being Fe and inevitable impurities, in which the relationship Ti+Nb×48/93≥8×(C+N) (in the relational expression, Ti, Nb, C, and N denote the contents (mass %) of the corresponding chemical elements) is satisfied.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT InternationalApplication No. PCT/JP2015/003735, filed Jul. 27, 2015, and claimspriority to Japanese Patent Application No. 2014-177911, filed Sep. 2,2014, the disclosures of each of these applications being incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a ferritic stainless steel sheet forthe casing of a urea SCR catalyst carrier.

BACKGROUND OF THE INVENTION

Urea SCR (SCR: Selective Catalytic Reduction) is an environmentaltechnology for making NOx harmless by reducing NOx into nitrogen (N₂)and water (H₂O) in the presence of a catalyst through the use of ammonia(NH₃) which is derived by decomposing aqueous urea and is used mainlyfor diesel powered trucks.

It is known that, in the process of deriving NH₃ by decomposing thisaqueous urea at a high temperature, a highly corrosive material calledammonium carbamate (NH₂COONH₄) is generated.

A conventional urea SCR catalyst carrier is composed mainly of ceramic,and such a ceramic urea SCR catalyst carrier having a circular cylinderform is wrapped with a ferritic stainless steel sheet which is used as acasing for supporting the carrier. Therefore, the inner side (the sidefacing the ceramic) of such a ferritic stainless steel sheet is requiredto be resistant to corrosion caused by ammonium carbamate. For example,Patent Literature 1 discloses a technique in which a ferritic stainlesssteel sheet which contains 10.0 mass % to 20.0 mass % of Cr and whichhas been subjected to dip pickling in nitric acid is used for the partsof a urea SCR system.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No.2012-112025

SUMMARY OF THE INVENTION

Here, it is supposed that, since diesel powered trucks equipped with aurea SCR system are used for various purposes ranging fromshort-distance transportation in a town to long-distance transportation,the amount of corrosive ammonium carbamate generated varies depending onthe temperature of the exhaust gas and the amount of the exhaust gas. Inaddition, it is thought that, in the case where a large amount ofammonium carbamate is generated, the corrosion of a ferritic stainlesssteel sheet tends to progress.

From this point of view, in the case of the ferritic stainless steelsheet according to Patent Literature 1, there is a problem of severecorrosion occurring in a urea SCR casing due to its insufficientcorrosion resistance against ammonium carbamate in some cases.Therefore, there is a demand for a ferritic stainless steel sheet for aurea SCR casing having higher corrosion resistance.

Therefore, an object of the present invention is to provide a ferriticstainless steel sheet for a urea SCR casing excellent in terms ofcorrosion resistance in a urea SCR environment.

The present inventors, in order to solve the problem described above,conducted investigations regarding the influences of various alloyelements on corrosion resistance in a urea environment by performing aHuey test, which is used for evaluating a material for a ureamanufacturing plant, and, as a result, found that it is possible toobtain a steel sheet excellent in terms of corrosion resistance whichcan preferably be used for a urea SCR casing by adding 20.5 mass % ormore of Cr, 0.40 mass % or more of Cu, and an appropriate amount of Tior Nb to a steel sheet.

The subject matter of the present invention includes the following.

[1] A ferritic stainless steel sheet for a urea SCR casing, the steelsheet having a chemical composition containing, by mass %, C: 0.020% orless, Si: 0.01% or more and 0.50% or less, Mn: 0.01% or more and 0.50%or less, P: 0.040% or less, S: 0.010% or less, Al: 0.01% or more and0.20% or less, Cr: 20.5% or more and 24.0% or less, Cu: 0.40% or moreand 0.80% or less, Ni: 0.05% or more and 0.6% or less, N: 0.020% orless, one or both selected from among Ti: 0.01% or more and 0.40% orless and Nb: 0.01% or more and 0.55% or less, and the balance being Feand inevitable impurities, in which the relationship Ti+Nb×48/93≥8×(C+N). . . (1)

(in relational expression (1), Ti, Nb, C, and N denote the contents(mass %) of the corresponding chemical elements) is satisfied.

[2] The ferritic stainless steel sheet for a urea SCR casing accordingto item [1] above, the steel sheet having the chemical compositionfurther containing, by mass %, Mo: 0.01% or more and 0.25% or less.

[3] The ferritic stainless steel sheet for a urea SCR casing accordingto item [1] or [2] above, the steel sheet having the chemicalcomposition further containing, by mass %, one, two, or all of V: 0.01%or more and 0.20% or less, Zr: 0.01% or more and 0.20% or less, and Ca:0.0002% or more and 0.0020% or less.

The ferritic stainless steel sheet for a urea SCR casing according tothe present invention is excellent in terms of corrosion resistance in aurea SCR environment and capable of increasing the durability of a ureaSCR casing.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The ferritic stainless steel sheet for a urea SCR casing according tothe present invention has a chemical composition containing, by mass %,C: 0.020% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.01% ormore and 0.50% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.01%or more and 0.20% or less, Cr: 20.5% or more and 24.0% or less, Cu:0.40% or more and 0.80% or less, Ni: 0.05% or more and 0.6% or less, N:0.020% or less, one or both selected from among Ti: 0.01% or more and0.40% or less and Nb: 0.01% or more and 0.55% or less, and the balancebeing Fe and inevitable impurities, in which the relationshipTi+Nb×48/93≥8×(C+N) (in the relational expression, Ti, Nb, C, and Ndenote the contents (mass %) of the corresponding chemical elements) issatisfied. The ferritic stainless steel sheet for a urea SCR casingaccording to the present invention is excellent in terms of corrosionresistance against ammonium carbamate and capable of increasing thedurability of a urea SCR casing.

Hereafter, the reasons for the limitations on the chemical compositionof the ferritic stainless steel sheet for a urea SCR casing according tothe present invention (hereinafter, referred to as “the steel sheetaccording to the present invention”) will be described. Here, “%”related to the constituent chemical elements in the steel sheetaccording to the present invention denotes “mass %”, unless otherwisenoted.

[C: 0.020% or Less]

Since C is a chemical element which decreases the workability andtoughness of a steel sheet, and since such negative effects becomenoticeable in the case where the C content is more than 0.020%, the Ccontent is limited to be 0.020% or less. In particular, from theviewpoint of workability and toughness, it is preferable that the Ccontent be 0.017% or less, or more preferably 0.012% or less.

[Si: 0.01% or More and 0.50% or Less]

Si is a chemical element which is necessary as a deoxidizing agent. Suchan effect is realized in the case where the Si content is 0.01% or more.However, since there is a decrease in pickling capability by forming aSiO₂ film on the surface of a steel sheet as a result of being oxidizedwhen cold-rolled-sheet annealing is performed, the upper limit of the Sicontent is set to be 0.50%. It is preferable that the Si content be0.20% or less from the viewpoint of pickling capability.

[Mn: 0.01% or More and 0.50% or Less]

Mn is a chemical element which is necessary as a deoxidizing agent. Suchan effect is realized in the case where the Mn content is 0.01% or more.However, since there is a decrease in the workability of a steel sheetin the case where the Mn content is excessively large, the Mn content islimited to be 0.50% or less. It is preferable that the Mn content be0.30% or less, or more preferably 0.20% or less, from the viewpoint ofthe workability of a steel sheet.

[P: 0.040% or Less]

Since P is a chemical element which decreases the workability andtoughness of a steel sheet, it is preferable that the P content be assmall as possible, and the P content is set to be 0.040% or less, orpreferably 0.030% or less.

[S: 0.010% or Less]

Since S is a chemical element which decreases toughness, it ispreferable that the S content be as small as possible, and the S contentis set to be 0.010% or less. It is preferable that the S content be0.006% or less from the viewpoint of toughness.

[Al: 0.01% or More and 0.20% or Less]

Al is a chemical element which is effective as a deoxidizing agent. Suchan effect is realized in the case where the Al content is 0.01% or more.However, since there is a decrease in surface quality as a result of,for example, scab flaws occurring due to an excessive amount ofAl₂O₃-based inclusions being formed in the case where the Al content isexcessively large, the Al content is limited to be 0.20% or less. It ispreferable that the Al content be 0.05% or less from the viewpoint ofsurface quality.

[Cr: 20.5% or More and 24.0% or Less]

Cr is a chemical element which is effective for increasing corrosionresistance against ammonium carbamate in a urea SCR environment, whichcharacterizes the present invention, because it suppresses theoccurrence of corrosion by strengthening a passivation film, which isformed on the surface of a stainless steel sheet. Since it is necessarythat the Cr content be 20.5% or more in order to achieve sufficientcorrosion resistance, the lower limit of the Cr content is set to be20.5%. It is more preferable that the Cr content be 21.0% or more. Onthe other hand, since Cr decreases the toughness of a steel sheet, andsince there is a significant decrease in toughness, in particular, inthe case where the Cr content is more than 24.0%, the Cr content islimited to be 24.0% or less. It is preferable that the Cr content be22.0% or less from the viewpoint of toughness.

In addition, the present inventors found that, by adding 20.5% or moreand 24.0% or less of Cr and, as described below, 0.40% or more and 0.80%or less of Cu simultaneously to a steel sheet, since Cr suppresses theoccurrence of corrosion, and since Cu suppresses the progress ofcorrosion, a steel sheet has significantly excellent corrosionresistance against ammonium carbamate from the synergy of such effects.

[Cu: 0.40% or More and 0.80% or Less]

Cu is a chemical element which has a function of suppressing theprogress of pitting corrosion (the progress of corrosion) in the casewhere pitting corrosion occurs due to corrosion and which is accordinglyeffective, in particular, of increasing corrosion resistance againstammonium carbamate in a urea SCR environment. Such an effect is realizedin the case where the Cu content is 0.40% or more. On the other hand,since there is a decrease in toughness in the case where the Cu contentis more than 0.80%, the Cu content is set to be 0.80% or less. It ispreferable that the Cu content be 0.60% or less from the viewpoint oftoughness.

[Ni: 0.05% or More and 0.6% or Less]

Ni is effective for increasing corrosion resistance and toughness, andsuch effects are realized in the case where the Ni content is 0.05% ormore. However, since Ni increases material costs, the Ni content is setto be 0.6% or less.

[N: 0.020% or Less]

Since N is, like C, a chemical element which decreases workability andtoughness, and since such negative effects become noticeable in the casewhere the N content is more than 0.020%, the N content is limited to be0.020% or less. In particular, it is preferable that the N content be0.015% or less, or more preferably 0.012% or less, from the viewpoint ofworkability and toughness.

[One or both selected from among Ti: 0.01% or more and 0.40% or less andNb: 0.01% or more and 0.55% or less] and [Ti+Nb×48/93≥8×(C+N)]

Ti and Nb inhibit a sensitization phenomenon, in which the corrosionresistance is decreased due to the combination of Cr with carbon andnitrogen, as a result of forming carbonitrides. Such an effect isrealized in the case where the Ti content or the Nb content is 0.01% ormore and where

Ti+Nb×48/93≥8×(C+N)   (1)

is satisfied (in relational expression (1), Ti, Nb, C, and N denote thecontents (mass %) of the corresponding chemical elements). In addition,from the viewpoint of inhibiting sensitization, it is preferable thatthe Ti content be 0.20% or more or that the Nb content be 0.30% or morewhile relational expression (1) is satisfied. However, in the case wherethe Ti content or the Nb content is excessively large, the effect ofinhibiting sensitization becomes saturated, and there is a decrease intoughness. Therefore, the Ti content is set to be 0.40% or less, and theNb content is set to be 0.55% or less. It is preferable that the Ticontent be 0.35% or less and that the Nb content be 0.45% or less fromthe viewpoint of toughness.

In the case of the steel sheet according to the present invention, theremainder other than the constituent chemical elements described aboveis Fe and inevitable impurities.

In addition, in the present invention, Mo, V, Zr, and Ca may be addedfor the purposes described below and within the ranges described below,although these chemical elements are not essential constituent chemicalelements.

[Mo: 0.01% or More and 0.25% or Less]

Mo is a chemical element which is effective for increasing corrosionresistance, and such an effect is realized in the case where the Mocontent is 0.01% or more. However, since there is a decrease intoughness in the case where the Mo content is more than 0.25%, the Mocontent is set to be 0.01% or more and 0.25% or less in the case whereMo is added.

[V: 0.01% or More and 0.20% or Less]

V is a chemical element which increases workability in the case where Vis added. Such an effect is realized in the case where the V content is0.01% or more. However, since there may be a decrease in surface qualitydue to the occurrence of surface defects in the case where the V contentis more than 0.20%, the V content is set to be 0.01% or more and 0.20%or less in the case where V is added.

[Zr: 0.01% or More and 0.20% or Less]

Zr is a chemical element which increases workability in the case whereZr is added. Such an effect is realized in the case where the Zr contentis 0.01% or more. However, since there may be a decrease in surfacequality due to the occurrence of surface defects in the case where theZr content is more than 0.20%, the Zr content is set to be 0.01% or moreand 0.20% or less in the case where Zr is added.

[Ca: 0.0002% or More and 0.0020% or Less]

Since Ca has a deoxidizing effect, Ca is added as needed. In order torealize such an effect, the Ca content is set to be 0.0002% or more, orpreferably 0.0005% or more. However, in the case where the Ca content ismore than 0.0020%, the deoxidizing effect becomes saturated, and theremay be a decrease in surface quality due to scab flaws occurring as aresult of Ca forming inclusions. Therefore, in the case where Ca isadded, the Ca content is set to be 0.0020% or less.

[Manufacturing Method]

There is no particular limitation on what method is used formanufacturing the ferritic stainless steel sheet for a urea SCR casingaccording to the present invention as long as the chemical compositionof molten steel is controlled as described above at the molten steelstage, and a method which is generally used for manufacturing a ferriticstainless steel sheet may be used.

Preferable conditions for the manufacturing method described above willbe described hereafter.

In a steel-making process in which molten steel is prepared, it ispreferable to manufacture steel containing the essential constituentchemical elements described above and the constituent chemical elementswhich are added as needed by performing secondary refining on moltensteel, which has been prepared by using a converter, an electricfurnace, or the like, by using, for example, a VOD method. Although theprepared molten steel may be made into a steel material (slab) by usinga known method, it is preferable that a continuous casting method beused from the viewpoint of productivity and product quality.Subsequently, the steel material is heated to a temperature of 1000° C.to 1250° C. and then hot-rolled into a hot-rolled steel sheet having adesired thickness. It is needless to say that the steel material may beformed into shapes other than a sheet shape by performing hot working.The hot-rolled steel sheet obtained as described above may be subjectedto descaling by performing, for example, pickling after continuousannealing has been performed at a temperature of 850° C. to 1100° C. Inaddition, although there is no particular limitation on cooling rateafter annealing has been performed, it is preferable that the coolingtime be as short as possible. Here, scale may be removed as needed byperforming shot blasting before pickling is performed.

Moreover, the hot-rolled and annealed steel sheet or the hot-rolledsteel sheet described above may be made into a cold-rolled product byperforming processes such as a cold rolling process. In this case,although cold rolling may be performed only once, cold rolling may beperformed twice or more with interposing process annealing between coldrolling from the viewpoint of productivity and desired productqualities. It is preferable that the total rolling reduction of the coldrolling process, in which cold rolling is performed once, twice, ormore, be 60% or more, or more preferably 70% or more. It is preferablethat the cold-rolled steel sheet be made into a cold-rolled product bythen performing continuous annealing (finish annealing) at a temperatureof 850° C. to 1150° C., or more preferably 900° C. to 1100° C., andpickling. Also, in this case, although there is no particular limitationon a cooling rate after annealing has been performed, it is preferablethat the cooling rate be as large as possible. Moreover, in the case ofsome purposes of use, for example, skin pass rolling may be performedafter finish annealing has been performed in order to control the shape,surface roughness, and material quality of a steel sheet.

EXAMPLE 1

Hereafter, the present invention will be described on the basis ofexamples. As examples, cold-rolled steel sheets used as samples wereobtained by using the following methods.

By preparing steel ingots having a weight of 50 kg and the chemicalcompositions given in Table 1 by using a vacuum melting furnace, byheating the steel ingots to a temperature of 1200° C., by thenperforming hot rolling using a reverse rolling mill in order to obtainhot-rolled steel sheets having a thickness of 3 mm, by annealing thehot-rolled steel sheets at a temperature of 930° C. to 1100° C., and byperforming descaling through pickling the annealed steel sheets,hot-rolled and pickled steel sheets were obtained. Subsequently, byperforming cold rolling using a reverse rolling mill on the hot-rolledand pickled steel sheets in order to obtain a thickness of 1.0 mm, byperforming finish annealing at a temperature of 880° C. to 970° C., andby dipping the annealed steel sheets in a mixed acid (containing 10 mass% of nitric acid and 3 mass % of hydrofluoric acid) having a temperatureof 60° C. in order to remove scale, cold-rolled steel sheets wereobtained.

In order to investigate corrosion resistance in a urea SCR environment,by performing a test in accordance with JIS Z 0573 (Method of 65 percentnitric acid test for stainless steels) on samples, that is, thecold-rolled steel sheets described above, corrosion rate (g/m²/h) over48 hours was derived. A case where the corrosion rate was 0.35 g/m²/h orless was judged as a satisfactory case. The results are given in Table1.

TABLE 1 Chemical Composition (Unit: mass %) No. C Si Mn P S Al Cr Ni NTi Nb 1 0.0095 0.09 0.45 0.038 0.002 0.033 23.8 0.21 0.0070 0.35 — 20.0015 0.35 0.10 0.023 0.001 0.080 20.5 0.05 0.0168 0.35 — 3 0.0155 0.050.38 0.038 0.003 0.020 21.5 0.40 0.0051 0.29 0.06 4 0.0020 0.45 0.120.018 0.003 0.045 21.0 0.10 0.0144 0.25 — 5 0.0071 0.11 0.33 0.030 0.0030.028 21.2 0.21 0.0111 — 0.40 6 0.0089 0.13 0.16 0.029 0.001 0.038 20.60.25 0.0070 0.30 — 7 0.0092 0.15 0.15 0.031 0.002 0.030 20.8 0.23 0.00910.33 — 8 0.0113 0.09 0.12 0.028 0.003 0.033 21.4 0.21 0.0101 0.29 — 90.0080 0.11 0.16 0.029 0.002 0.030 20.8 0.22 0.0098 0.31 0.01 10 0.01100.12 0.15 0.019 0.002 0.031 20.7 0.19 0.0115 0.26 0.22 11 0.0189 0.050.48 0.021 0.008 0.021 23.5 0.17 0.0111 0.39 — 12 0.0101 0.47 0.10 0.0400.001 0.150 20.5 0.45 0.0191 0.35 — 13 0.0083 0.12 0.11 0.028 0.0020.033 20.6 0.21 0.0099 0.32 — 14 0.0095 0.15 0.15 0.025 0.001 0.035 19.30.15 0.0093 0.29 0.10 15 0.0091 0.14 0.19 0.031 0.002 0.031 20.7 0.230.0095 0.12 — 16 0.0113 0.11 0.18 0.028 0.003 0.028 20.6 0.16 0.0103 —0.25 Urea SCR Ti + Nb × Environment Chemical Composition (Unit: mass %)48/93 − Corrosion No. Mo Cu V Zr Ca 8 × (C + N) Test Result 1 — 0.53 — —— 0.22 Satisfactory Example 2 — 0.50 — — — 0.20 Satisfactory Example 3 —0.43 — — 0.0002 0.16 Satisfactory Example 4 — 0.40 0.05 — — 0.12Satisfactory Example 5 — 0.48 0.15 — 0.0008 0.06 Satisfactory Example 60.25 0.41 — — 0.0011 0.17 Satisfactory Example 7 0.15 0.44 0.08 — — 0.18Satisfactory Example 8 — 0.42 0.05 0.08 0.0012 0.12 Satisfactory Example9 0.08 0.44 0.03 — 0.0011 0.17 Satisfactory Example 10 — 0.42 0.03 —0.0009 0.19 Satisfactory Example 11 — 0.70 0.15 — — 0.15 SatisfactoryExample 12 — 0.78 0.08 — 0.0002 0.12 Satisfactory Example 13 — 0.36 0.05— 0.0009 0.17 Unsatisfactory Comparative Example 14 0.12 0.44 0.08 — —0.19 Unsatisfactory Comparative Example 15 — 0.41 0.03 — 0.0011 −0.03Unsatisfactory Comparative Example 16 — 0.40 0.05 — — −0.04Unsatisfactory Comparative Example

All the examples of the present invention were satisfactory in the ureaSCR corrosion test, which means these examples had excellent corrosionresistance. On the other hand, No. 13, whose Cu content was less thanthe range according to the present invention, No. 14, whose Cr contentwas less than the range according to the present invention, and No. 15and No. 16, whose Ti content and Nb content did not satisfy therelationship expressed by relational expression (1) according to thepresent invention, that is, “Ti+Nb×48/93≥8×(C+N)”, were unsatisfactoryin the urea SCR corrosion test. As described above, it is clarified thatthe steels according to the present invention are excellent in terms ofcorrosion resistance in a urea SCR environment.

As described above, since the steel sheet according to the presentinvention is excellent in terms of corrosion resistance in a urea SCRenvironment, the steel sheet can preferably be used as a material for acasing supporting a urea SCR catalyst carrier.

1. A ferritic stainless steel sheet for a urea SCR casing, the steelsheet having a chemical composition containing, by mass %, C: 0.020% orless, Si: 0.01% or more and 0.50% or less, Mn: 0.01% or more and 0.50%or less, P: 0.040% or less, S: 0.010% or less, Al: 0.01% or more and0.20% or less, Cr: 20.5% or more and 24.0% or less, Cu: 0.40% or moreand 0.80% or less, Ni: 0.05% or more and 0.6% or less, N: 0.020% orless, one or both selected from among Ti: 0.01% or more and 0.40% orless and Nb: 0.01% or more and 0.55% or less, and the balance being Feand inevitable impurities, wherein the relationshipTi+Nb×48/93≥8×(C+N)   (1) is satisfied, wherein in relational expression(1), Ti, Nb, C, and N denote the contents (mass %) of the correspondingchemical elements.
 2. The ferritic stainless steel sheet for a urea SCRcasing according to claim 1, the chemical composition furthercontaining, by mass %, Mo: 0.01% or more and 0.25% or less.
 3. Theferritic stainless steel sheet for a urea SCR casing according to claim1, the chemical composition further containing, by mass %, one, two, orall of V: 0.01% or more and 0.20% or less, Zr: 0.01% or more and 0.20%or less, and Ca: 0.0002% or more and 0.0020% or less.
 4. The ferriticstainless steel sheet for a urea SCR casing according to claim 2, thechemical composition further containing, by mass %, one, two, or all ofV: 0.01% or more and 0.20% or less, Zr: 0.01% or more and 0.20% or less,and Ca: 0.0002% or more and 0.0020% or less.